Introduction
Blood is a vital fluid for the body. The blood temperature is about one degree higher than the body temperature. The quantity of blood depends on the size and weight. The average individual has approximately five to six liters of blood. Blood has three primary functions: transportation (transports oxygen from the lungs to cells for metabolism and removal of carbon dioxide), regulation (maintenance of body temperature), and protection (platelets, white blood cells, and certain chemical messengers play a crucial role in the immune system).
For infection to enter the body, the organisms must overcome the skin, nose, mucus, and stomach acid (physical barriers; the first line of defense). Blood and its components comprise the second line of defense.
What Are Blood Components That Help in Immunity?
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The white blood cells (WBCs; also called leukocytes) comprise granulocytes (neutrophils, eosinophils, basophils, and mast cells; which contain granules released during infections) and agranulocytes (monocytes and lymphocytes; which do not contain granules). Lymphocytes mature in the bone marrow and the lymph nodes. The cells are released into the bloodstream after maturation. Then, they migrate to peripheral tissues. Also, they circulate in the blood and the lymphatic system (a specialized immune system).
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Further, cytokines (immune mediators) stimulate the production of WBCs. The WBCs are involved in innate (inherited) and adaptive (the response that develops after an infection) immunity. The granulocytes have a short life and are produced in increased numbers during an infection. During the process, they leave the blood to migrate to the inflammation (the response to an infection or injury) sites.
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Neutrophils are the most abundant and important cellular component of the innate immune response. Therefore, hereditary neutrophil deficiencies lead to bacterial infection.
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Eosinophils increase and become active in defense against parasitic infections and allergies.
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Mast cells reside near small blood vessels and release substances that affect blood vessel permeability. They are known for organizing allergic responses and protecting mucosal surfaces against pathogens.
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The function of basophils is similar to that of eosinophils and mast cells. Macrophages play a critical part in innate immunity as they are distributed throughout the body. Natural killer cells (also originating from lymphoid cells) lack antigen-specific receptors and are part of the innate immune system. These cells circulate in the blood as large lymphocytes with cytotoxic (cell-killing) granules. They can recognize and kill some tumor cells and virus-infected cells. Hence, they are elementary in the innate immune defense against pathogens.
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Dendritic cells (also called antigen-presenting cells) are specialized to pick an antigen and display it for lymphocyte recognition. After encountering a pathogen, they rapidly migrate to lymph nodes and mature. The lifespan of dendritic cells within draining lymph nodes is short, and optimal antigen presentation by such cells occurs within 24 hours of an infection.
What Is the Role of Platelets in Immunity?
Platelets lack a nucleus and originate from megakaryocytes (large bone marrow cells). They play the most crucial role in blood clotting. If a blood vessel is damaged, healing starts with the clumping of the platelets on the inside of the damaged blood vessel wall. As a result, it quickly causes a plug to close the wound temporarily. Simultaneously, strong protein threads are attached to hold the clump in place.
However, many diseases are not strictly associated with blood vessel damage. Still, they manifest as a tendency towards excessive clot formation or excessive bleeding. These include bacterial and parasite infections, cancers, and autoimmune disorders. Hence, these findings inspire new research regarding platelets as immune cells.
Platelets contain various surface receptors, glycoproteins, cytoskeletal elements, and granules. Upon activation, surface receptors enable platelets to recognize infection-causing pathogens and form immune complexes. Subsequently, changes occur in the composition and quantity of their surface proteins. As a result, activated platelets release the contents of their granules. These granules release cytokines (immune mediators) that help elicit an immune response.
Activated platelets also express different membrane glycoproteins. The glycoproteins bind to various receptors triggering an inflammatory response. Further, it leads to the release of leukocyte-attracting mediators. The membrane glycoproteins also regulate T-cell function and dendritic cell activation. They also provide a novel mechanism for platelet autoactivation and platelet aggregate formation. These properties make them perfect immune cells. Hence, platelets can be considered active regulators of immunity.
What Is the Role of Lymphocytes in Immunity?
Lymphocytes (T and B) (originate from lymphoid cells) mount a specific immune response against any foreign antigen (adaptive immunity). It is possible because each lymphocyte has a unique antigen (a foreign protein) receptor variant. Hence, they collectively bear highly diverse receptors. The B cell secretes antibodies (immune proteins) after activation and differentiation to plasma cells. Antibodies, also called immunoglobulins (Ig), and the antigen receptor of B lymphocytes are known as membrane immunoglobulin (mIg). The T-cell antigen receptor (TCR) is specially adapted to detect antigens or pathogens that enter host cells.
How Do Lymphocytes Generate an Immune Response?
Small B and T lymphocytes that have not yet encountered antigens are called naive lymphocytes. These cells enter the peripheral lymphoid tissues (lymph nodes and spleen) by squeezing between the cells of the capillaries. During an infection, lymphocytes proliferate and differentiate into effector cells to fight infection after identifying the infectious agent.
When an infection occurs in the periphery, large numbers of antigen travel from the infection site through lymphatic vessels into the draining lymph nodes. In the lymph nodes, dendritic cells display the antigen to T lymphocytes and also help in their activation. B cells encounter antigens as they migrate through the lymph nodes. Once the antigen-specific lymphocytes proliferate and differentiate, they leave the lymph nodes as effector cells.
The peripheral lymphoid tissues vary depending upon the presence or absence of an infection. It is because these lymphoid tissues may only appear in response to infection. For example, the B-lymphocyte follicles of the lymph nodes expand when the B lymphocytes proliferate to form germinal centers.
Conclusion
The blood cells and plasma constituents interact in a complex manner to confer immunity against infectious agents. Further, they resist or destroy invading organisms, produce an inflammatory response, and remove foreign materials. Thorough knowledge of blood and its relation with immunity paves the way for the evolution of new treatment regimes targeting the immune system. It is exciting to note that developments in the field of blood components are changing innate and adaptive immune responses.
